(1) Field of the Invention
This invention relates to a method for the manufacture of miniature micro probes or electrical contacts for use in testing semiconductor chips.
(2) Description of the Related Art
It is known in the art of testing probe cards for electrical continuity to perform such tests using probes made by mechanically forming a straight piece of fine wire into a desired shape so as to provide the necessary size and spring force. FIGS. 1-3 show a conventional “Cobra™” probe test head produced by Wentworth Laboratories, Inc. of Brookfield, Conn. Such probe heads consist of an array of probes 64 held between opposing first (upper) 42 and second (lower) 44 dies. Each probe has opposing upper and lower ends. The upper and lower dies 42, 44 contain patterns of holes corresponding to spacing on an integrated circuit contact pad spacing designated herein as lower die hole pattern and upper die hole pattern. The upper end of each of the probes is retained by the upper die hole pattern, and the lower end of each of the probes passes through the lower die hole pattern and extends beyond the lower die 44 to terminate in a probe tip. With reference to FIG. 13, there is illustrated the additional inclusion of mounting film 1301. Mounting film 1301 is typically formed from a suitable polymeric dielectric such as mylar and holds the etched probes 81 in place. For Cobra™ style probes, the lower die hole pattern is offset from that in the upper die 42, and the offset is formed into the probe such that the probe acts like a spring. Returning to FIGS. 1-3, when the test head is brought into contact with a wafer to be tested, the upper end of the probe remains predominately stationary, while the lower end compresses into the body of the test head. This compliance allows for variations in probe length, head planarity, and wafer topography. The probe is typically formed by swaging or stamping a straight wire to produce the desired probe shape and thickness. This swaging process flattens and widens the center, curved portion of the probe in order to achieve a desired force per mil of probe deflection.
The lower and upper ends of the swaged area also prevent the probe from extending too far through the dies. In a conventional probe manufacturing process, the probes are formed from a straight piece of wire, typically of beryllium-copper alloy. Custom tooling is used for each probe size and design. The tooling stamps and forms the center portion of the wire to achieve the desired shape and thickness, thereby generating a desired spring rate.
With reference to FIG. 9 there is illustrated cross sectional renderings of a wire used in the prior art to produce probes. Cross section 90 illustrates the generally circular form of the pre-stamped wire. Cross section 91 illustrates the generally elliptical shape of a stamped and tooled wire. The cross sectional areas of both cross section 90 and cross section 91 are substantially the same. With reference to cross section 91, the stamped wire forming the probe has a width 95 of approximately 7 mil (one mil equals 0.001 inch) and a height 97 of approximately 1.8 mils. When assembled in a probe head configuration it is preferable to maintain at least a 1 mil separation between the plurality of probes used in the probe head. As a result of width 95 being approximately 7 mils and requiring a 1 mil separation, conventional probes arranged in a probe head are typically spaced one probe every 8 mils. The wire is then cut to length, and the desired probe tip geometry is ground on the end of the probe. The tolerance on the overall length of the finished probes is +/−0.002″. Because this is too large a variance between probes for proper testing, the probes are assembled into a probe head and the entire array of probes is lapped to achieve a more uniform probe length.
Conventional stamping processes used to form probes often result in residual stresses in the probes which may cause reduced fatigue life. Because these residual stresses can change over time, changes in probe stiffness may arise. In addition, changes in the requirements for probes require retooling. Such retooling contributes to a high cost for probes manufactured in such a fashion and require a substantial lead time before such probes are available. It is also the case that mechanically fashioned probes are more difficult to redesign as their construction is closely tied to the mechanical means by which they are created.
There therefore exists a need for a method of manufacturing such probes that avoids the problems which arise from mechanical formation. There is further a need for such a method substantially amenable to producing probes of different designs absent a protracted retooling process.